1. Field of the Invention
The present invention relates to an electromagnetic actuator.
2. Description of Related Art
An electromagnetic actuator includes a coil, a plunger, and a stator. The coil generates magnetic force when energized. The plunger is driven by magnetic force generated by the coil. The stator forms a magnetic circuit. A driven object (e.g., valve) is driven by displacement of the plunger (e.g., JP2003-97756A).
The stator forms the magnetic circuit along inner and outer circumferences of the coil through the plunger. The stator is formed by combining components to receive the coil inside the stator.
More specifically, a conventional stator described in JP2003-97756A, for example, is formed by joining a magnetic attraction portion, a radial direction magnetism delivery-receipt portion, and a cylindrical yoke. The magnetic attraction portion attracts the plunger in its axial direction. Magnetism in a radial direction of the plunger is delivered and received between the plunger and the radial direction magnetism delivery-receipt portion. The yoke covers an outer circumference of the coil.
In the stator in JP2003-97756A and the like, magnetism is delivered and received at a joining portion of the magnetic attraction portion and the yoke, and magnetism is delivered and received at a joining portion of the radial direction magnetism delivery-receipt portion and the yoke. That is, in the conventional stator, magnetism is delivered and received at two joining portions.
Since magnetic loss by a contact resistance is caused at the joining portion, magnetic force generated at the stator decreases due to the joinings at two portions. Accordingly, there is a problem that attraction of the plunger is decreased.
The magnetic attraction portion and the yoke are joined by joining an outer circumferential portion of a flange portion having a disc-like ring shape, which is formed at the magnetic attraction portion, and an inner circumferential surface of the yoke, respectively. The radial direction magnetism delivery-receipt portion and the yoke are joined by joining an outer circumferential portion of a flange portion having a disc-like ring shape, which is formed at the radial direction magnetism delivery-receipt portion, and the inner circumferential surface of the yoke, respectively. That is, in the conventional stator, the components are joined at the respective outer circumferential portions of the flange portions, which are small areas.
Magnetic loss is large because of a small joining area, so that magnetic force generated at the stator decreases and attraction of the plunger is decreased. Furthermore, magnetic saturation is easily reached because of a small joining area, so that magnetic flux of the stator is difficult to increase and attraction of the plunger is prevented from increasing.
Thus, due to the small joining area as well as the joinings at two portions in the conventional stator of the electromagnetic actuator, magnetic loss is large and attraction of the plunger is prevented from being improved.